Thus the first week of embryogenesis takes place within the lumens of the oviduct and uterus. For most of this time the embryo remains encapsulated by the zona pellucida.
The first two cleavages take about 2 days, and the embryo reaches a 16-cell morula by 3 days. The outer cells of the morula become tightly adhesive with each other and begin transporting fluid into the embryonic mass. During days 4 and 5 the transport of fluid generates a cavity called the blastocyst cavity, and the embryo is now called a blas
Fig. 44.33 ). The blastocyst is composed of two subpopulations of cells: an eccentric inner cell mass and an outer epithelial-like layer of trophoblasts. The region of the trophoblast layer immediately adjacent to the inner cell mass is referred to as the embryonic pole, and it is this region that attaches to the uterine endometrium at
Fig. 44.33
The embryo resides within the oviduct during the first 3 days and then enters the uterus. By 5 to 6 days of develop ment the trophoblasts of the blastocyst secrete proteases that digest the outer-lying zona pellucida. At this point, corresponding to about day 22 of the menstrual cycle, the “hatched” blastocyst is able to adhere to and implant into the receptive uterine endometrium (see
Fig. 44.33
At the time of attachment and implantation the tro phoblasts differentiate into two cell types: an inner layer of cytotrophoblasts and an outer layer of multinuclear/
Fig. 44.33).
The cytotrophoblasts initially provide a feeder layer of continu ously dividing cells. Syncytiotrophoblasts initially perform three general types of function: adhesive, invasive, and endocrine. Syncytiotrophoblasts express adhesive surface proteins (i.e., cadherins and integrins) that bind to uterine surface epithelia and, as the embryo implants, to compo nents of the uterine extracellular matrix. In humans the embryo completely burrows into the superficial layer of the endometrium (see
Fig. 44.33). This mode of implanta tion, called interstitial implantation, is the most invasive among placental mammals. Invasive implantation involves adhesion-supported migration of syncytiotrophoblasts into the endometrium, along with the breakdown of extracel lular matrix by secretion of matrix metalloproteinases and other hydrolytic enzymes.
The endocrine function begins with the onset of implantation, when syncytiotrophoblasts start secreting the LH-like protein hCG, which maintains the viability of